Catalysts for Oxidation of Carbon Monoxide and/or Volatile Organic Compounds

ABSTRACT

This application discloses catalysts and methods of making the catalysts. In one embodiment, a catalyst comprising: a reduced precious group metal in an amount greater than about 30 wt % based on the total precious group metal weight in the catalyst, wherein the catalyst oxidizes volatile organic compounds and/or carbon monoxide at a temperature of about 150° C. or lower, is disclosed. In another embodiment, a catalyst for oxidation of formaldehyde, methanol, formic acid, and/or carbon monoxide to form carbon dioxide at a temperature of from about 20° C. to about 45° C. and at about atmospheric pressure, the catalyst comprising: a reduced precious group metal dispersed on a support selected from the group consisting of CeO 2 , TiO 2 , ZrO 2 , Al 2 O 3 , SiO 2 , and combinations thereof, is disclosed.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to U.S. Provisional Patent ApplicationNo. 61/869,137, filed on Aug. 23, 2013, which is incorporated herein byreference in its entirety.

FIELD OF THE INVENTION

This invention relates to catalysts, compositions comprising thecatalysts, methods of making the catalysts, and uses of the catalystsfor oxidation of carbon monoxide (CO) and/or volatile organic compounds(VOCs) to carbon dioxide at temperatures that are less than about 150°C., and in certain embodiments at ambient temperatures. In someembodiments, the catalyst oxidizes the VOCs and/or CO at ambienttemperatures ranging from about 15° C. to about 30° C.

BACKGROUND OF THE INVENTION

Industrial operations typically emit large quantities of pollutants suchas CO and VOCs. Vehicles, aircrafts, waste water treatment plants, lightmanufacturing facilities, certain small businesses (e.g., dry cleaners,bakeries, restaurants, etc.), and homes also emit CO and VOCs, albeittypically in much smaller quantities compared to industrial operations.

CO is known for its toxicity to humans and animals due to its highaffinity to hemoglobin, which reduces the effectiveness of oxygentransportation in blood even at a concentration level of a hundred ppm.VOCs cause several health and environmental problems and are alsoprecursors of ground-level ozone, which contributes to smog formation.The overall chemistry is a complex interaction between VOCs, NOx andozone which results in the formation of photochemical smog. Conventionaltechnologies such as themocatalytic oxidation are usually found to beexpensive to implement and have a tendency to result in secondarypollution at low temperatures.

The emission control of VOCs and CO at temperatures ranging from about20° C. to about 50° C. has become increasingly important for publichealth, government regulation, and business development. For example,formaldehyde which is a major indoor air pollutant has been listedrecently as a carcinogen. Currently known technologies for VOCs and COabatement at low temperatures, especially at about room temperature,include photocatalysis, high voltage discharge, sorbents, and oxidationcatalysts.

However, none of the currently known methods appear to achieve removalof VOCs and/or CO at temperatures ranging from about 20° C.-50° C. bycomplete oxidation.

From a practical application point of view, removal of VOCs and/or CO attemperatures ranging from about 20° C. to about 50° C. by completeoxidation has significant advantages over other known methods due to itslow consumption of energy and raw materials, low production cost, andhigh selectivity.

There is a need to develop more active oxidation catalysts that allowthe complete oxidation of CO and the VOCs at room temperature withsufficient stability and durability.

SUMMARY OF THE INVENTION

According to one embodiment, a catalyst comprising: a reduced preciousgroup metal in an amount greater than about 30 wt % based on the totalprecious group metal weight in the catalyst, wherein the catalystoxidizes VOCs and/or CO at a temperature of about 150° C. or lower, isdisclosed.

In certain embodiments, the catalyst oxidizes the VOCs and/or CO atambient temperatures ranging from about 15 to about 30° C.

According to another embodiment, a catalyst for oxidation of VOCs and/orCO to form carbon dioxide at a temperature of from about 20° C. to about45° C. and at about atmospheric pressure, the catalyst comprising: areduced precious group metal dispersed on a support selected from thegroup consisting of CeO₂, TiO₂, ZrO₂, Al₂O₃, SiO₂, and combinationsthereof, is disclosed.

According to another embodiment, a method of making a precious groupmetal catalyst comprising: (i) impregnating a precious group metal on asupport in the form of a dissolved salt solution; and (ii) reducing theprecious group metal in cationic form to a reduced precious group metalin metallic form by reductants in gas phase, liquid phase, solid phase,or combinations thereof, is disclosed.

The oxidation of CO and/or VOCs such as formaldehyde, methanol, andformic acid to form carbon dioxide at a temperature of from about 20° C.to about 45° C. and at about atmospheric pressure can be completed at aconversion of about 90% or higher. In certain embodiments, the oxidationof formaldehyde, methanol, formic acid, and/or CO to form carbon dioxideat a temperature of from about 25° C. to about 35° C. and at aboutatmospheric pressure can be completed at a conversion of about 95% orhigher. In certain embodiments, the oxidation of formaldehyde, methanol,formic acid, and/or CO to form carbon dioxide at a temperature of fromabout 25° C. to about 35° C. and at about atmospheric pressure can becompleted at a conversion of about 98% or higher. In certainembodiments, the oxidation of formaldehyde, methanol, formic acid,and/or CO to form carbon dioxide at a temperature of from about 25° C.to about 35° C. and at about atmospheric pressure can be completed at aconversion of about 99% or higher. In certain embodiments, the oxidationof formaldehyde, methanol, formic acid, and/or CO to form carbon dioxideat a temperature of from about 25° C. to about 35° C. and at aboutatmospheric pressure can be completed at a conversion of about 100%.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a chart of CO and CO₂ concentration profiles during COoxidation on 1% Pt/TiO₂ at 25° C.

FIG. 2 shows a chart of the dependence of CO conversion on waterconcentration at 25° C.

FIG. 3 shows a chart of the conversion of formaldehyde, methanol, andformic acid on 1% Pt/TiO₂ catalyst at 25° C.

FIG. 4 shows a chart of oxidation of formaldehyde and methanol on 1%Pd/TiO₂ at 25° C. (CO: 500 ppm; HCHO: 123 ppm; CH3OH: 166 ppm; SV:50,000/h; 1% Pd/TiO2).

FIG. 5( a) shows a chart of catalyst stability in the presence of CO₂for oxidation of CO at 25° C.

FIG. 5( b) shows a chart of catalyst stability in the presence of CO₂for oxidation of formaldehyde and methanol on 1% Pt/TiO2 at 25° C.

FIG. 6( a) shows a Transmission Electron Microscopy (TEM) of 1% Pt/TiO2.

FIG. 6( b) shows a TEM of 1% Pd/TiO2.

DETAILED DESCRIPTION

The terms “about” or “approximately” when used herein and associatedwith a numeric value refer to that numeric value plus or minus 10%, incertain embodiments plus or minus 5%, in certain other embodiments plusor minus 2%, in yet certain other embodiments plus or minus 1%, in yetother embodiments plus or minus 0.5%, and in some other embodiments plusor minus 0.1%.

“Complete oxidation” as used herein refers to oxidation of CO and VOCssuch as formaldehyde, methanol, and formic acid to form carbon dioxideat a temperature of from about 20° C. to about 45° C. and at aboutatmospheric pressure at a conversion of about 90% or higher, in someembodiments about 95% or higher, in some other embodiments about 98% orhigher, in yet other embodiments about 99% or higher, and in certainembodiments about 100%.

The oxidation or complete oxidation of CO and VOCs referred to in thisapplication is carried out at atmospheric pressure and at a temperatureof less than about 150° C., in some embodiments at a temperature rangingfrom about 0° C. to about 100° C., in some other embodiments at atemperature ranging from about 15° C. to about 50° C., in yet otherembodiments at a temperature ranging from about 20° C. to about 30° C.,and in certain embodiments at a temperature ranging from about 21° C. toabout 28° C.

According to one embodiment, a catalyst comprising: a reduced preciousgroup metal in an amount greater than about 30 wt % based on the totalprecious group metal weight in the catalyst, wherein the catalystoxidizes VOCs and/or CO at a temperature of about 150° C. or lower, isdisclosed.

In certain embodiments, the catalyst oxidizes the VOCs and/or CO atambient temperatures ranging from about 15 to about 30° C.

The reduced precious group metal is present in the catalyst in an amountbetween about 30 wt % and about 95 wt % based on the total preciousgroup metal weight in the catalyst. In certain embodiments, the reducedprecious group metal is present in the catalyst in an amount betweenabout 50 wt % and about 95 wt % based on the total precious group metalweight in the catalyst. In certain other embodiments, reduced preciousgroup metal is present in the catalyst in an amount between about 60 wt% and about 95 wt % based on the total precious group metal weight inthe catalyst.

The reduced precious group metal has a mean crystallite size (i.e., meandiameter) of about 3 nm or less. In certain embodiments, the reducedprecious group metal has a mean crystallite size in the range of about 2nm or less. In certain other embodiments, the reduced precious groupmetal has a mean crystallite size in the range of about 1 nm to about 2nm.

The reduced precious group metal is selected from the group consistingof platinum, palladium, rhodium, ruthenium, iridium, gold, and mixturesthereof. In certain embodiments, the reduced precious group metal isplatinum.

The reduced precious group metal is dispersed on a support selected fromthe group consisting of CeO₂, ZrO₂, TiO₂, SiO₂, Al₂O₃, clay, zeolite,and mixtures thereof. In certain embodiments the support is CeO₂ orTiO₂.

In some embodiments, the reduced precious group metal can be dispersedon a support selected from the group consisting of a porous polymer,activated carbon, cellulose, wood powder, and mixtures thereof.

In other embodiments, the reduced precious group metal is dispersed on acomposite material support, inorganic support, organic support, orcombinations thereof.

According to another embodiment, a catalyst for oxidation offormaldehyde, methanol, formic acid, and/or CO to form carbon dioxide ata temperature of from about 20° C. to about 45° C. and at aboutatmospheric pressure, the catalyst comprising: a reduced precious groupmetal dispersed on a support selected from the group consisting of CeO₂,TiO₂, ZrO₂, Al₂O₃, SiO₂, and combinations thereof, is disclosed.

The oxidation of formaldehyde, methanol, formic acid, and/or CO to formcarbon dioxide at a temperature of from about 20° C. to about 45° C. andat about atmospheric pressure can be completed at a conversion of about90% or higher. In some embodiments, the oxidation of formaldehyde,methanol, formic acid, and/or CO to form carbon dioxide at a temperatureof from about 25° C. to about 35° C. and at about atmospheric pressurecan be completed at a conversion of about 95% or higher. In some otherembodiments, the oxidation of formaldehyde, methanol, formic acid,and/or CO to form carbon dioxide at a temperature of from about 25° C.to about 35° C. and at about atmospheric pressure can be completed at aconversion of about 98% or higher. In yet other embodiments, theoxidation of formaldehyde, methanol, formic acid, and/or CO to formcarbon dioxide at a temperature of about from about 25° C. to about 35°C. and at about atmospheric pressure can be completed at a conversion ofabout 99% or higher. In certain other embodiments, the oxidation offormaldehyde, methanol, formic acid, and/or CO to form carbon dioxide ata temperature of from about 25° C. to about 35° C. and at aboutatmospheric pressure can be completed at a conversion of about 100%.

The reduced precious group metal in certain embodiments is platinum.

In some embodiments, the catalyst can be promoted by bismuth oxide. Theweight ratio of bismuth to the reduced precious group metal is betweenabout 0.1 and about 100. In some embodiments, the weight ratio ofbismuth to the reduced precious group metal is between about 1 and about75. In some other embodiments, the weight ratio of bismuth to thereduced precious group metal is between about 1 and about 10.

According to another embodiment, a method of making a precious groupmetal catalyst comprising: (i) impregnating a precious group metal on asupport in the form of a dissolved salt solution; and (ii) reducing theprecious group metal in cationic form to a reduced precious group metalin metallic form by reductants in gas phase, liquid phase, solid phase,or combinations thereof, is disclosed.

The reductants in gas phase are hydrogen or formic acid. The reductantsin liquid or solid phase are formic acid, ammonium formate, or any otherknown organic or inorganic reducing agents such as ascorbic acid andhydrazine.

Step (i) in the above process can be completed by incipient wetness,rotary evaporation, spray drying, or combinations thereof.

The catalyst obtained by the above process can be coated on a monolithhoneycomb, a formed refractory oxide substrate, a formed polymersubstrate, and combinations thereof.

There are several application of the catalysts described herein. Forexample, the catalysts described herein can be used for vehicle andaircraft cabin air cleaning, addressing “sick building syndrome” inhomes and buildings, cleaning CO and VOCs emanating from municipalunderground pipes, coal mines, etc.

The catalysts described herein have been demonstrated to remove up toabout 100% of CO and VOCs at temperatures ranging from about 20° C. toabout 30° C. by complete oxidation to CO₂. The catalysts describedherein have also demonstrated good stability in the presence of waterand CO₂.

Several special features of the presently described catalysts relatingto preparation, reaction conditions, and activity are discussedhereinbelow.

The Pt-based catalysts are reduced at a relatively low temperature priorto the reaction for the formation of finely dispersed precious metalcrystallites with platinum, which is in metallic state in certainembodiments.

Adding metal oxide promoters such as bismuth oxide enhancessignificantly the oxidation conversion due mainly to the stabilizationof precious metal dispersion. Bismuth oxide may also reduce the surfaceoccupation of Pt sites by CO. Of the Pt catalysts studied in this work,Bi-promoted Pt/CeO₂ showed the highest activity.

For CO oxidation, the presence of a small amount of moisture as low asabout 500 ppm in the reactant mixture promotes the oxidation greatly.

In certain embodiments, at least about 500 ppm of water is added to thereactant mixture to enhance the complete oxidation of CO.

On the other hand, presence of moisture had some negative effect for theoxidation of VOCs, such as methanol.

In certain embodiments, moisture is removed from the reactant mixtureprior to its contact with catalysts for the complete oxidation of VOCs.

The following reactions show complete oxidation of CO and VOCs to carbondioxide.

2CO+O₂→2CO₂

HCOH+O₂→CO₂+H₂O

2CH₃OH+3O₂→2CO₂+4H₂O

CH₃OOH+O₂→CO₂+2H₂O

EXAMPLES 1. Catalyst Synthesis

Catalysts were prepared by a simple impregnation of platinum nitrate ora mixture of platinum nitrate and bismuth nitrate aqueous solution on anoxide support, dehydration in a rotary evaporator at 80° C. followed bydrying at 110° C. overnight, and then reduction in 5% H₂/N₂ at 400° C.for up to 2 hours. Typical Pt loading is 1 wt %, though the PM loadinglevel can be adjusted depending on the applications and the reactionconditions required.

2. Catalyst Testing

Catalyst activity was measured using a flow-through reactor. 10 grams ofpelletized catalyst sample with a particle size of 40-60 mesh was placedin the quartz tube reactor of 1 inch diameter. A typical space velocitywas 50,000 hr⁻¹. Because of different bulk density of various supportsused, to keep the space velocity constant, some samples were dilutedwith pelletized alumina to make the catalyst volume the same. Thestarting concentration of CO, formaldehyde, methanol, and formic acidwere approximately 500, ˜100, ˜140, and ˜150 ppm, respectively. Thegaseous reactants (either from a gas tank mixed with N₂ or by bubblingN₂ through an organic liquid) were mixed with air prior to entering thereactor. Typical O₂ concentration in the reactor was about 20%. Thereaction products were identified and quantified by a FTIR detector (MKSMultiGas 2030). To study the effect of CO₂ and water, a controlledamount of CO₂ and/or 2.0% water were added to the reactant mixture whenneeded.

3. Catalyst Characterization

Precious metal morphology and crystallite size was characterized by highresolution TEM and X-ray diffraction (XRD). Precious metal oxidationstate and speciation was determined by X-ray photoelectron spectroscopy(XPS). Some instrumental information for each spectroscopy method isgiven below.

TEM data was collected on a JEOL JEM2011 200 KeV LaB6 source microscopewith a Bruker Ge EDS system using Spirit software. Digital images werecaptured with a bottom mount Gatan 2K CCD camera and Digital Micrographcollection software. All powder samples were prepared and analyzed asdry dispersions on 200 mesh lacey carbon coated Cu grids.

XRD—a PANalytical MPD X′Pert Pro diffraction system was used withCu_(Kα) radiation generator settings of 45 kV and 40 mA. The opticalpath consisted of a ¼° divergence slit, 0.04 radian soller slits, 15 mmmask, ½° anti-scatter slits, the sample, 0.04 radian soller slits, Nifilter, and a PIXCEL position sensitive detector. The samples were firstprepared by grinding in a mortar and pestle and then backpacking thesample (about 2 grams) into a round mount. The data collection from theround mount covered a range from 10° to 90° 2θ using a step scan with astep size of 0.026° 2θ and a count time of 600 s per step. A carefulpeak fitting of the XRD powder patterns was conducted using Jade Plus 9analytical XRD software. The phases present in each sample wereidentified by search/match of the PDF-4/Full File database from ICDD,which is the International Center for Diffraction Data. Crystallite sizeof PdO was estimated through whole pattern fitting (WPF) of the observeddata and Rietveld refinement of crystal structures.

XPS—spectra were taken on a Thermo Fisher K-Alpha XPS system which hasan aluminum K Alpha monochromatic source using 40 eV pass energy (highresolution). Samples were mounted on double sided tape under a vacuum ofless than 5×10-8 torr. Scofield sensitivity factors and Avantagesoftware were used for quantification.

Results

1. Catalytic Activity

1-1 CO Oxidation.

FIG. 1 demonstrates the FTIR intensities of CO and CO₂ during theoxidation reaction of CO on 1% Pt/TiO₂ at room temperature. There wasabout 600 ppm CO₂ in the starting reactant mixture as a background. Thekinetics of CO oxidation was fast: 100% CO conversion to CO₂ wasobserved as soon as CO and CO₂ were mixed together and 1100 ppm of CO₂was detected. FIG. 1 only covers a small section of the reaction time upto 100 minutes. The CO conversion remained 100% during the entirereaction time of 48 hours.

1-2 Effect of Pre-Reduction on CO Oxidation.

Pre-reduction of the Pt catalysts was found to be critical for the COoxidation reaction. For example, when the 1% Pt/TiO₂ catalyst was onlycalcined in air at 400 or 550° C., the CO conversion was about 9% withthe absence of moisture and 11% with the presence of moisture. After thesample was reduced at 400° C., CO conversion was greatly improved to100%, same as the sample that was reduced directly without theprecalcination in air, see Table 1.

TABLE 1 CO conversion (%) to CO₂ over reduced, calcined, and calcined &reduced 1% Pt/TiO₂ catalysts at room temperature Reduced CalcinedCalcined & Reduced w/o w/ w/o w/ w/o w/ H₂O H₂O H₂O H₂O H₂O H₂O 100 1009 11 40 100

1-3 Water Promotion on CO Oxidation.

Water played a significant role in CO oxidation reaction. FIG. 2demonstrates the dependence of CO conversion on water concentration over1% Pt/CeO₂ catalyst. The data shows that a very small amount of watercould dramatically enhance the CO conversion at a water concentrationbetween 0 and 500 ppm. The conversion remained 100% after H₂Oconcentration was above 500 ppm. Similar effect of water concentrationwas observed for 1% Pt/TiO₂ catalyst.

1-4 Effect of Bismuth Additive on CO Oxidation.

To evaluate the effect of bismuth additive on catalyst activity, anumber of reactor conditions were varied including space velocity SV, O₂concentration [O₂], and CO concentration [CO] on four catalysts: 1%Pt/TiO₂, 1% Pt/CeO₂, (1% Pt+2.5% Bi)/TiO₂, and (1% Pt+2.5% Bi)/CeO₂. Theresults are listed in Table 2.

TABLE 2 Effect of reactor conditions on CO conversion (%) to CO₂ Otherreactor 1% Pt on (1% Pt + 2.5% 1% Pt on (1% Pt + 2.5% Reactor variablesconditions TiO₂ Bi)/TiO₂ CeO₂ Bi)/CeO₂ SV =  50 k [CO] = 500 ppm, 100100 100 100 100 k [O₂] = 20%, 100 100 100 100 200 k [H₂O] = 2% 100 10096 100 500 k 100 100 90 100 [O₂] = 20% SV = 1,000,000 hr⁻¹, 40 92 64 9410% [CO] = 500 ppm; 25 88 50 92  5% [H₂O] = 2% 10 75 46 88  1% 5 34 3664 0.2%  2 16 28 20 [CO] =  500 ppm SV = 1,000,000 hr⁻¹, 32 84 44 882000 ppm [O₂] = 20%; 10 25 21 50 5000 ppm [H₂O] = 2% 8 20 20 24  1% 4 1012 10

The data in Table 2 shows that, in a fairly wide range of space velocitybetween 50,000 to 500,000 hr⁻¹, CO conversion was at or close to 100% ata constant [CO]=500 ppm, [O2]=20%, and [H2O]=2%. On the other hand, at aspace velocity of 1 million hr⁻¹, significantly reduced CO conversionwas observed when O₂ concentration is below 10% or CO concentration wasabove 2000 ppm. The relative activity of the four catalysts is in thefollowing order:

(1% Pt+2.5% Bi)/CeO₂>(1% Pt+2.5% Bi)/TiO₂>1% Pt/CeO₂>1% Pt/TiO₂

1-5 VOC Oxidation.

FIG. 3 demonstrates the conversion of formaldehyde, methanol, and formicacid on 1% Pt/TiO₂ catalyst at room temperature. The reaction profilesin FIG. 3 were divided into five sections referred hereinafter asSection I, Section II, Section III, Section IV, and Section V. InSection I, formaldehyde and methanol showed 100% conversion when waterwas absent in the reactant mixture. A small perturbation of theconversion at the first 5-10 minutes was due to theabsorption/desorption of formaldehyde or methanol on the catalyst. Afterwater was introduced (Section II), the conversion of formaldehyderemained 100% while the conversion of methanol was reduced to about 70%.However, when water and formaldehyde were removed from the reactantmixture (Section III), the conversion of methanol was recovered to 100%.Water had little effect on the conversion of formic acid, whichapproached to 100% regardless of whether water was present (Section IV)or absent (Section V).

Table 3 summarizes the conversion rate of CO, formaldehyde, methanol,and formic acid on 1% Pt catalysts supported on various oxides with bothpresence and absence of water. [O₂]=20%, SV=20,000 hr⁻¹ for VOCs and50,000 hr⁻¹ for CO, starting concentration [HCOH]˜100 ppm, [CH₃OH]˜140ppm, [CH₃OOH]˜150 ppm, [CO]=500 ppm, and [H₂O]=2%

TABLE 3 Conversion (%) for oxidation of CO and VOC under ambientconditions (i.e., about 25° C. and atmospheric pressure) 1% PtFormaldehyde Methanol Formic Acid CO catalyst w/ w/o w/ w/o w/ w/o w/w/o Support H₂O H₂O H₂O H₂O H₂O H₂O H₂O H₂O TiO₂ 100 100 70 100 100 100100 40 ZrO₂ 100 100 80 98 100 100 100 60 Al₂O₃ 100 100 65 96  NT* NT 10040 CeO₂ 100 100 70 40 100 100 100 100 TiO₂ 100 100 25 100 NT NT 100 100(2.5% Bi) *NT = not tested

The main conclusions from the above experimental data on Pt catalystscan be summarized in the following.

For CO oxidation in the absence of moisture, when a single componentsupport was used, e.g., TiO₂, ZrO₂, Al₂O₃, or CeO₂, only 1% Pt/CeO₂ gavea 100% conversion. However, adding a small amount of water significantlypromoted the conversion to 100% for all the Pt catalysts.

Pre-reduction of the Pt catalysts is crucial for CO oxidation at roomtemperature. When 1% Pt/TiO₂ catalyst was only calcined in air, verylittle CO oxidation was observed. Reduction of the calcined samples ledto 100% of CO conversion, same as those reduced directly withoutcalcination in air.

Adding promoters such as bismuth also significantly enhanced the COoxidation. For example, an addition of 2.5% Bi to 1% Pt/TiO₂ increasedthe conversion of CO oxidation from 40% to 100% with the absence ofwater.

For the oxidation of formaldehyde, all the catalysts showed a 100%conversion after 2 hours of reaction time regardless of whether or notmoisture was present in the reactant mixture. 100% oxidation of formicacid was also observed for all the catalysts that were studied, i.e., 1%Pt supported on TiO2, ZrO2, and CeO2. Methanol was more challenging tobe fully oxidized to CO₂, especially when moisture was present. Waterhas a major negative effect on the methanol conversion.

1-6 Activity of Other Precious Metals.

Table 4 shows the conversion of CO and formaldehyde over other preciousmetal catalysts supported on TiO₂ under the ambient conditions (i.e.,about 25° C. and atmospheric pressure). Palladium is a lower costprecious metal compared to platinum. When supported on TiO₂, the Pdcatalyst showed 100% conversion of CO with the presence of waterregardless whether the catalyst was reduced or calcined in air at 400°C. When water was removed, the CO conversion decreased slowly to about60% after 1,000 minutes of reaction time. Pd/TiO₂ also showed 100%conversion for formaldehyde, whether water was present or not, similaras Pt/TiO₂. However, there was no conversion of methanol at roomtemperature for Pd/TiO₂, regardless whether moisture was present or not.The observation of some “conversion” at the first 30 minutes of reactiontime, shown in FIG. 4, was due to adsorption and desorption of methanolby the catalyst. The conversion of formaldehyde on Ir/TiO₂ and PtRu/TiO₂are also listed in Table 4.

TABLE 4 CO and formaldehyde oxidation under ambient conditions (i.e.,about 25° C. and atmospheric pressure) over precious metal catalysts(other than Pt) supported on TiO₂ Conversion, Catalyst Reactant %Conditions 1% Pd/ Formaldehyde 100 123 ppm HCHO, 20% O₂ in TiO₂ or air,0 or 2% H₂O, 50 k SV 1% PdO/ CO 100 500 ppm CO, 20% O₂ in air, TiO₂ 2%H₂O, 50 k SV 1% Ir/ Formaldehyde 98 120 ppm HCHO, 20% O₂ in TiO₂ air, 2%H₂O, 50 k SV 1%(Pt + Formaldehyde 100 105 ppm HCHO, 20% O₂ in Ru,Pt/air, 2% H₂O, 50 k SV Ru = 1)/ TiO₂

2. Catalysts Stability

As all the reactions were studied at room temperature, the preciousmetals and the oxide supports do not experience the aging effect asthose used at high temperature. It was reported that water and CO₂ hadpoisoning effect when catalysts were exposed to air. As shown in Table3, we found that water actually helps the CO oxidation. Water had somenegative impact on the oxidation of methanol, possibly due tocompetitive adsorption of water and methanol on the precious metalsites. CO₂ did not show any significant impact on the oxidation of CO,formaldehyde, and methanol, shown in FIGS. 5( a) and 5(b).

Analysis 1. Effect of Support Porosity

In order to understand the relationship between catalyst structures andtheir activity, a number of physical and chemical properties of thecatalysts were studied. The N₂ porosity of the support materials wasmeasured, and the BET surface area (BET) and pore volume (PV) are listedin Table 5. Although the surface area and pore volume were variedsignificantly among the support oxides, after loaded with the platinum,the activity of the catalysts were all high and were not apparently tocorrelate with the support porosity.

TABLE 5 N₂ Porosity of the support materials BET, PV*, Material m²/gcc/g TiO₂ 85 0.55 CeO₂ 148 0.34 ZrO₂ 15 0.14 Al₂O₃ 156 0.84 *PV = porevolume, PD = pore diameter

2. Effect of Precious Metal Crystallite Size.

The crystallite size of dispersed platinum and palladium was measured byXRD and TEM. As the catalysts were prepared at a mild condition (thehighest heating temperature was 400° C.), the PM crystallites size onthe supports were small in general. TEM shows that Pt or Pd particlesize on TiO₂ and CeO₂ is about 1-2 nm (FIGS. 6( a) and 6(b)). The Ptparticle size on CeO₂ with Bi additive is too small to be detected byTEM, possibility in the sub-nanometer range. Thus, the high activity ofPt catalysts is most likely due to the high Pt dispersion. The higher COconversion on bismuth promoted catalysts might be due to the smaller Ptsize.

3. Effect of Surface Pt Concentration and Oxidation State

XPS data offers further details of Pt surface concentration andoxidation state. The data in Table 6 shows the ionic Pt species andnitrate in the dried-only sample, which is consistent with its zeroactivity. After being calcined in air at 400° C., nitrate species wereburned off and Pt were mainly in the form of Pt(+2) or Pt(+4). No Pt(0)species was detected. On the other hand, when the catalyst was reducedeither from the dried sample or the pre-calcined sample, the dominant Ptspecies are Pt(0). No nitrate species was detected. When comparing theXPS data in Table 6 with the CO oxidation data in Table 1, it is clearthat it is the Pt(0) that was responsible for CO conversion. In otherwords, it is essential to keep platinum in highly dispersed metallicstate for the complete oxidation of CO.

TABLE 6 XPS speciation of Pt/TiO₂ pre-treat under different conditionsSample Treatment Pt(0) Pt(+2) Pt(+4) NO₃ Pt(0), % Dried @ 110° C. ND*0.38 0.07 1.2 0 Reduced in H₂ @ 400° C. 0.25 0.13 ND ND 66 Calcined inair @ 400° C. ND 0.28 0.13 ND 0 Calcined in air @ 400 C 0.20 0.09 ND ND69 and then reduced in H₂ @ 400° C. *ND = not detectable

Low temperature removal of CO and small VOC molecules such asformaldehyde, methanol, and formic acid is highly desirable for publichealth protection. The above examples demonstrate that these pollutantscan be removed 100% at ambient temperatures (i.e., ranging from about 15to about 30° C.) by complete oxidation to CO₂ over precious metal basedcatalysts. The catalysts also showed good stability in the presence ofwater and CO₂. Several features of catalyst preparation and the reactionconditions proved to be important, as discussed previously.

The catalysts should be useful for the removal of other VOC species suchas alkanes, alkenes, alcohols, aldehydes, ketones, amines, organicacids, aromatic compounds, and/or combinations thereof typicallyencountered as air pollutants.

The present invention has been described by way of the foregoingexemplary embodiments to which it is not limited. Variations andmodifications will occur to those skilled in the art that do not departfrom the scope of the invention as recited in the claims appendedthereto.

What is claimed is:
 1. A catalyst comprising: a reduced precious groupmetal in an amount greater than about 30 wt % based on the totalprecious group metal weight in the catalyst, wherein the catalystoxidizes volatile organic compounds and/or carbon monoxide at atemperature of about 150° C. or lower.
 2. The catalyst of claim 1,wherein the reduced precious group metal is present in the catalyst inan amount between about 30 wt % and about 95 wt % based on the totalprecious group metal weight in the catalyst.
 3. The catalyst of claim 1,wherein the reduced precious group metal has a mean crystallite size ofabout 3 nm or less.
 4. The catalyst of claim 1, wherein the reducedprecious group metal is selected from the group consisting of platinum,palladium, rhodium, ruthenium, iridium, gold, and mixtures thereof. 5.The catalyst of claim 1, wherein the catalyst oxidizes volatile organiccompounds and/or carbon monoxide at temperatures ranging from about 15°C. to about 30° C.
 6. The catalyst of claim 1, wherein the reducedprecious group metal is dispersed on a support selected from the groupconsisting of CeO₂, ZrO₂, TiO₂, SiO₂, Al₂O₃, clay, zeolite, and mixturesthereof.
 7. The catalyst of claim 1, wherein the reduced precious groupmetal is dispersed on a support selected from the group consisting of apolymer, activated carbon, cellulose, wood powder, and mixtures thereof.8. The catalyst of claim 1, wherein the reduced precious group metal isdispersed on a composite material support, inorganic support, organicsupport, or combinations thereof.
 9. A catalytic system comprising thecatalyst of claim
 1. 10. A catalyst for oxidation of volatile organiccompounds and/or carbon monoxide to form carbon dioxide at a temperatureof from about 20° C. to about 45° C. and at about atmospheric pressure,the catalyst comprising: a reduced precious group metal dispersed on asupport selected from the group consisting of CeO₂, TiO₂, ZrO₂, Al₂O₃,SiO₂, and combinations thereof.
 11. The catalyst of claim 10, whereinthe oxidation of the volatile organic compounds and/or the carbonmonoxide to form carbon dioxide at a temperature of from about 20° C. toabout 45° C. and at about atmospheric pressure is completed at aconversion of about 90% or higher.
 12. The catalyst of claim 10, whereinthe reduced precious group metal is selected from the group consistingof platinum, palladium, rhodium, ruthenium, iridium, gold, and mixturesthereof.
 13. The catalyst of claim 10, wherein the catalyst is promotedby bismuth oxide.
 14. The catalyst of claim 13, wherein the weight ratioof bismuth to the reduced precious group metal is between about 0.1 toabout
 100. 15. A method of making a precious group metal catalystcomprising: (i) impregnating a precious group metal on a support in theform of a dissolved salt solution; and (ii) reducing the precious groupmetal in cationic form to a reduced precious group metal in metallicform by reductants in gas phase, liquid phase, solid phase, orcombinations thereof.
 16. The method of claim 15, wherein the reductantsin gas phase are hydrogen or formic acid.
 17. The method of claim 15,wherein the reductants in liquid or solid phase are formic acid,ammonium formate, or any other known organic and inorganic reducingagents including ascorbic acid and/or hydrozine.
 18. The method of claim15, wherein step (i) is completed by incipient wetness, rotaryevaporation, spray drying, or combinations thereof.
 19. The catalystobtained from the method of claim
 15. 20. The catalyst of claim 19 iscoated on a monolith honeycomb, a formed refractory oxide substrate, aformed polymer substrate, or combinations thereof.
 21. The catalyst ofclaim 19 removes and/or oxidizes carbon monoxide and/or volatile organiccompounds including alkanes, alkenes, alcohols, aldehydes, ketones,amines, organic acids, aromatic compounds, or combinations thereof, froma medium.
 22. The catalyst of claim 21 wherein the medium is air orwater.
 23. The catalyst of claim 19, wherein at least about 500 ppm ofwater is added to a reactant gas mixture to promote oxidation of carbonmonoxide and/or to remove moisture from the reactant gas mixture tomaximize oxidation of volatile organic compounds.
 24. The catalyst ofclaim 1, wherein the oxidation of the volatile organic compounds and/orthe carbon monoxide to form carbon dioxide at a temperature of fromabout 20° C. to about 45° C. and at about atmospheric pressure iscompleted at a conversion of about 90% or higher.
 25. The catalyst ofclaim 24, wherein the conversion is about 95% or higher.
 26. Thecatalyst of claim 10, wherein the volatile organic compounds includeformaldehyde, methanol, and/or formic acid.